https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html#tagtcjh_24 specifies the existance and semantics of the above identifiers. However, they are missing from glibc. I'd expect the ..._LDFLAGS to be -pthread or (standards-conforming) -lpthread and ..._CFLAGS to be -pthread. See also https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=649164